1. Field of the Invention
The present invention relates to a method for manufacturing a silicon carbide semiconductor device and, in particular, to a silicon carbide semiconductor device including a gate insulating film.
2. Description of the Background Art
A semiconductor element formed of silicon carbide (SiC) has recently attracted attention as a next generation high-withstand voltage, low-loss, high-heat resistant switching element, and is expected to be applied to a power semiconductor device, such as an inverter.
A vertical metal-oxide-semiconductor field-effect transistor (MOSFET) is well known as the power semiconductor device. When the MOSFET is formed of silicon carbide, a gate insulating film formed, for example, of silicon dioxide is required to be formed on a silicon carbide layer. The silicon dioxide film can be formed on the silicon carbide layer by a method of thermally oxidizing a surface part of the silicon carbide layer and a method of depositing the silicon dioxide film on the silicon carbide layer. In each of the methods, an interface state is formed at the interface between the silicon carbide layer and the silicon dioxide film. The interface state can act to make field-effect mobility of the MOSFET (SiC-MOSFET) formed of silicon carbide lower than that of silicon carbide bulk, increase resistance (on-resistance) during on-operation of the SiC-MOSFET, and consequently increase the loss of the SiC-MOSFET.
As a method for inactivating the interface state formed at the interface between silicon carbide and silicon dioxide, a method of, after formation of silicon dioxide on silicon carbide, treating them at a high temperature in a nitric oxide (NO) gas or a nitrous oxide (N2O) gas is known. For example, Japanese Patent No. 4911263 discloses technology for nitriding the interface between silicon carbide and silicon dioxide by heating a substrate manufactured through formation of the silicon dioxide film on the silicon carbide layer to a temperature of 1200° C. to 1300° C. inclusive so that the substrate reacts with nitrogen. In particular, in Japanese Patent No. 4911263, a flow rate of a mixed gas of nitrogen and nitric oxide (with a mixing ratio of 1:1 to 9:1) introduced into a furnace is set to 0.75 slm or higher when an inside diameter of the furnace is 140 mm, and a position of the substrate is adjusted so that the mixed gas is retained in the furnace for 50 seconds or shorter. As a result, partial pressure of oxygen generated by reaction between nitric oxide and nitrogen is controlled to be 0.05 or lower of partial pressure of nitric oxide at a position near the substrate.
The on-resistance of the SiC-MOSFET is expected to be reduced with the technology disclosed in Japanese Patent No. 4911263. However, the SiC-MOSFET still has a high on-resistance compared to a MOSFET (Si-MOSFET) formed of silicon, and thus further reduction of the on-resistance is desired.